This application claims priority from German application no. 201 19 513.5 filed Dec. 3, 2001.
The present invention relates to a reducing agent dosing device for delivering a reducing agent into the exhaust gas system of the internal combustion engine of a motor vehicle. The internal combustion engine has fuel transported in a fuel tank and a fuel return line is disposed between the internal combustion engine and the fuel tank. The reducing agent dosing device has a reducing agent tank and a supply line connecting the reducing agent tank with the exhaust gas system of the internal combustion engine.
Apart from carbon monoxide (CO) and hydrocarbons (HC), nitric oxides (NOx) are among the environmentally harmful, directly emitted, primary injurious substances which are generated during the operation of internal combustion engines, particularly diesel engines. Three-way catalysts, which are used in Otto engines and gas engines, cannot be used in the exhaust gas of diesel engines due to an oxygen excess. For this reason, to reduce nitric oxide emission in diesel engines a selectively operating SCR catalyst (Selective Catalytic Reduction Catalyst) has been developed in which, in the presence of an added reducing agent, namely ammonia (NH3), the expelled nitric oxides are reduced to N2 and H2O.
The ammonia required for carrying out the reduction can be transported along on board of the motor vehicle in different forms. The pure ammonia can be in the gaseous or also in the liquid phase. To avoid problems in handling the pure ammonia, it is preferably stored in bound form on the motor vehicle, for example in a tank. The ammonia carried in the bound form is hydrolytically split either in the exhaust gas system or preceding it to release the bound ammonia. One such a reducing agent is an aqueous solution of urea. The urea solution is stored in a reducing agent tank. The tank is connected over a supply line to the exhaust gas system of the internal combustion engine.
A dosing valve positioned on the exhaust gas system serves to deliver the required quantity of urea. The temperatures obtained in the exhaust gas system immediately gasify the urea, releasing the ammonia required for carrying out the nitric oxide reduction.
Due to the disposition of the dosing valve immediately on the exhaust gas system in the know prior art, the input side of the dosing valve must be cooled to prevent the liquid urea solution decomposing or crystallizing out due to the high temperatures. Such reducing agent dosing devices are known in the art, for example from DE 198 56 366 C1.
According to a further known implementation the dosing valve is disposed directly beneath the reducing agent tank and is connected with the exhaust gas system over a delivery line that is several meters long. Compressed air from the air compressor for the brake system transports the quantity of urea solution in the delivery line. The dosed urea solution quantity is, therefore, transported in the supply line as an aerosol.
There are known disadvantages in using of an aqueous urea solution as the reducing agent. One disadvantage is that the reducing agent dosing devices can only operate as specified at temperatures above the freezing point of the urea solution. Therefore, an aqueous urea solution can only be used as reducing agent only with additional heating devices. It is also necessary to heat inter alia the supply line used for the transport of the reducing agent in a liquid or an aerosol to prevent temperatures below the freezing point of the aqueous urea solution. Resistance heaters are conventionally used as heaters. Other heat providers can only be utilized with considerable expenditure for heating the supply line extending between the reducing agent tank and the exhaust gas system. For example, the cooling water, which becomes warm during the operation of the internal combustion engine, could be used. But the use of resistance heaters entails expenditures, since it must be ensured that overheating is avoided.
Building on this discussed prior art, the present invention addresses the problem of developing a reducing agent dosing device in which the supply line can be heated to temperatures, at least in the relevant sections during operation of the device, without having to tolerate the disadvantages demonstrated to be entailed in prior art.
This problem is solved according to the invention by having the supply line disposed such that it is connected with the fuel return line of the internal combustion engine in such manner that it is conducting heat.
The primary aspect of the present invention is to provide a reducing agent dosing device with a headed supply line.
Other aspects of this invention will appear from the following description and appended claims, reference being made to the accompanying drawings forming a part of this specification wherein like reference characters designate corresponding parts in the several views.
The reducing agent dosing device of the present invention has a supply line for transporting the reducing agent from a reducing agent tank to the exhaust gas system which is in heat-conducting connection to the fuel return line. This allows the heat contained in the fuel return line, which, as a rule, does not exceed 60-70xc2x0 C. during operation of the internal combustion engine, to heat the supply line, or at least its critical sections. It is especially advantageous that normally the fuel tank and the reducing agent container are disposed adjacent to one another. Therefore, the course of the supply line is usefully disposed parallel to the fuel return line. Both lines can be combined in a double tube developed as a double-walled tube with two concentric channels or also as channels extending parallel to one another. Consequently, the expenditure for the adequate heating of the supply line is reduced to a minimum. In particular, no additional regulation of the heat is required since the decomposition temperature of the urea, transported optionally in aqueous form in the supply line, is never exceeded.
Another advantage is that cooling of the fuel transported back in the fuel return line can also take place such that further cooling measures for cooling the returning fuel are in principle not necessary. The required heat in the fuel return line is available after an extremely short operating time of the internal combustion engine. In particular, the heat is available substantially earlier than when using cooling water, which must generally be heated through longer operation of the internal combustion engine.
Since the fuel return line extends from the fuel tank to the internal combustion engine, it is readily possible to heat substantially the entire supply line between the reducing agent tank and the dosing valve, in the event that the latter is disposed in the proximity of the exhaust gas system or the internal combustion engine for the output of the desired quantity of urea.
In a preferred embodiment of the reducing agent dosing device the dosing valve is located in direct association with the internal combustion engine. A liquid aqueous urea solution is present at the input side of the dosing valve. The dosing valve itself terminates in a delivery line supplied with compressed air. The delivery line is usefully supplied with compressed air by the charge air of the charging group of the internal combustion engine (for example turbocharger or compressor). One advantage of this system is that the dosing valve is disposed in the immediate proximity of the internal combustion engine and consequently is also heated by it. Additionally, the dosing valve is not positioned directly on the exhaust gas system such that this would necessitate cooling.
Utilizing compressed air as the transport medium for transporting the dosed quantity of urea in the form of an aerosol has the advantage that the quantity of air required for transporting the reducing agent output by the dosing valve is only very small in comparison to the quantity of air made available by the turbocharger and this branching-off of air is without further effects on the specified operation of the turbocharger. In particular when employing charge air in the described manner, the charge air can be removed before or after the charging-air cooler, such that air of different temperatures is available for mixing.